טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentTanya Glozman
SubjectIntegrated Ultrasonic Imaging Combining Computed Tomography
and Elastography
DepartmentDepartment of Biomedical Engineering
Supervisor Professor Azhari Haim
Full Thesis textFull thesis text - English Version


Abstract

The elastic properties of soft tissues depend on their molecular building blocks and on the microscopic and macroscopic organization of these blocks [14]. Pathological changes in tissue alter its mechanical properties, thus changing its stiffness . This is the rational behind the standard medical practice of soft tissue palpation, which provides a basic screening tool for qualitative assessment of tissue stiffness .   Many tissue pathologies, specifically cancers, are manifested by stiff localized lesions surrounded by normal soft tissue . However, since the echogenicity (the ability to cause sonographic contrast) and stiffness of tissues are caused by unrelated mechanisms, such lesions are not ultrasonically detectable in general .

Over the past two decades an increasing research effort focused on development and improvement of techniques for imaging tissue stiffness . This field is commonly referred to as Elastography . Estimation of the viscoelastic attributes of tissue can be achieved by straining the tissue and measuring the incurred displacements by any of the known imaging modalities. Among these, ultrasound imaging is one of the most widely used diagnostic tools nowadays due to its relatively low cost, safety and high temporal resolution.

The goal of this work was to devise a comprehensive ultrasound-based imaging method capable of measuring elastic parameters by combining both back-scattered elastography and through-transmitted ultrasonic computed tomography (UCT). Our suggested technique provides measurements of both longitudinal and shear wave velocities. This enables the non-invasive computation of several tissue elasticity parameters such as the Young and Shear moduli, Poisson's ratio and more importantly, the Bulk modulus, which can be determined only by measuring both wave velocities.

Four different phantom types were examined: agar-gelatin based phantoms, porcine fat tissue, turkey breast tissue and bovine liver tissue specimens. The values of the Young's modulus, Shear modulus and Poisson's ratio were estimated and were consistent with values published in the literature. The corresponding average Bulk modulus values were: 2.83±0.001 GPa, 2.25±0.01 GPa, 2.48±0.01 GPa and 2.53±0.02 GPa respectively. A statistically significant difference (p<0.001) in the values of the Bulk modulus of the different phantoms was found. This fact suggests that this parameter is most suitable for differentiation between different tissue types and pathological states. These results prove the feasibility of using a comprehensive ultrasound imaging technique for non-invasive remote quantitative palpation of tissues.